The present invention generally relates to devices and methods for flow measurement. In particular, a method for continuous multiphase flow measurement is disclosed, as well as the apparatus utilized in the method. In this invention, phase separation and measurement are accomplished utilizing a compact piping arrangement as compared to the measurement vessels utilized in conventional measurement systems.
Industry utilizes or has proposed several methods to measure the production of individual oil wells. The conventional approach is to use a three-phase or two-phase separator to separate the multi-phase fluid mixture into distinctive phases. In the case where a three-phase separator is employed, three separate outgoing streams are produced. These streams comprise gas, water, and oil (which may include water or comprise an oil/water emulsion). Separate flow meters measure the respective flow rates of the outgoing streams of oil, water, and gas. An on-line “cut” meter may be utilized to determine the water content of the oil stream. The two-phase separator operates similarly to the three-phase separator except that the free water stream is omitted.
These test separators are relatively large in physical size, expensive to construct, and require an abundance of ancillary pressure control and flow regulating equipment. Accordingly, it is rarely practical to utilize this approach for production testing an individual oil well. Instead, the practice is to utilize a single test separator to production test a group of wells. Each individual well is production tested for a relatively short period of time, and its production is determined. After the well is removed from test, it is assumed that the production from the well does not vary substantially until the well is again placed on test. Depending upon the number of wells serviced by the test separator, individual wells may be tested for a relatively short duration on a limited basis. This characteristic of multiple-well test systems may result in questionable well test results, and delay the detection of a problem with a particular well.
U.S. Pat. No. 5,390,547 (Liu) teaches a multiphase flow measurement method and apparatus using a piping arrangement which may be utilized for testing an individual well. Liu describes a technique for measuring flow rates for a multiphase fluid flow for continuously and respectively measuring the quantities of one gas and one or two liquid components flowing concurrently in a common pipeline. In Liu, the mixture delivered by a feed pipeline is separated into two separate streams of gas and liquid by a piping configuration, as opposed to conventional separators. The system then measures the flow rate in each stream individually. If there are multiple liquid components in the liquid phase, an on-line liquid fraction meter determines the proportion of each liquid component. The piping system then combines the two flow streams to a common discharge pipeline. Thus, Liu provides a technique to determine respective flow rates in a multiphase fluid flow system that is continuous and accurate using an apparatus, which is compact, low cost, reliable, and requires little maintenance. This technique has been effective in the measurement of oil and gas from individual wells.
The invention disclosed by Liu utilizes a two phase measurement technique, in that the incoming multiphase flow stream (i.e., natural gas, crude oil, and produced water) are separated into two separate streams, namely a gas phase and a liquid phase. The gas phase predominantly comprises natural gas. The liquid phase predominantly comprises a mixture of crude oil and produced water, but no further separation of the liquid phase occurs. Instead, a “water cut meter” is used to determine the water content in the liquid stream. The respective volumes of net oil and produced water are ascertained by applying the water cut measurement to the total liquid flow rate.
However, as the water cut in the liquid stream increases, the net oil measurement loses accuracy because the net oil measurement resolution sharply decreases when the water cut of the production stream increases. An increase in water cut is common for mature, depleting water drive hydrocarbon reservoirs.